1. Field of the Invention
This invention relates to a rolling/sliding member such as the outer ring of a cam follower apparatus in the valve drive mechanism of an engine.
2. Description of the Prior Art
Automobile engines include a crank shaft cooperating with a cam shaft having a cam for the valve drive mechanism.
The valve drive mechanism is used to transmit the motion of the cam, attached to the cam shaft, to the valves as the cam shaft rotates with the engine's crank shaft.
In recent years, it has been widely practiced to equip the valve drive mechanism with a cam follower apparatus, so that when the valve drive mechanism is in operation, the friction that occurs is transformed from sliding friction to rolling friction, thus keeping the friction loss to a minimum.
This kind of cam follower apparatus has an outer ring which is placed in a rolling and/or sliding contact with the mating parts or members opposed to it during operation.
In order to improve the durability of the surface of the rolling and/or sliding member, many inventions have been proposed in the past with respect to the improvement of the surface condition.
Japanese Patent Publication KOKAI NO. H1-30008 discloses an invention relating to a bearing roller having a rolling surface of which the surface roughness, Rmax, is between 0.3 to 1.5 .mu.m, and which is formed with scratches in random directions, and a surface layer having a residual stress of 50 kgf/mm.sup.2 or more.
Also, Japanese Patent First Publications KOKAI No. H3-117723 to No. H3-117725 disclose an invention relating to forming many depressions at random on the rolling surface using barrel processing, and in addition making the hardness of the surface layer harder than the inside or bulk hardness, such that residual compressive stress is caused to occur in the surface layer.
Moreover, Japanese Patent First Publication KOKAI No. H3-199716 discloses a bearing, where the surface that contacts the mating parts is hardened using a surface hardening process, and where the depth of the peak value of the residual compression stress is equal to the depth of the peak value of the shear stress distribution.
Also, Japanese Patent First Publication KOKAI No. H4-54312 discloses an invention relating to bearing parts, where the residual compressive stress, formed by using a shot peening process, is 100 kgf/mm.sup.2 or more on the surface, and the stress is 40 kgf/mm.sup.2 or more at 300 .mu.m below the surface.
Japanese Patent Publication KOKOKU No. H2-17607 discloses a surface processing method where shots are provided in the range of 40 to 200 .mu.m in size to have a hardness equal or higher than the hardness of the metal product, and the shots are projected over the surface of the metal product at a speed equal to or higher than 100 m/sec so as to increase the temperature around the surface of the product higher than the As transformation point.
In these aforementioned inventions, depending on the operating conditions, excellent durability is obtainable. However, when these inventions are applied to the rolling and/or sliding members which are used in conditions of poor lubrication in a rolling and/or sliding contact with the mating parts, it is impossible to obtain sufficient durability.
For example, when these inventions are applied to a cam follower apparatus which has an outer ring in a rolling and/or sliding contact relationship with the outer peripheral surface of the cam while the engine is running, peeling may occur, shortening the life of the follower.
In OHC-type and DOHC-type engines, where the valve drive mechanism is placed on top of the engine, the supply of lubricating oil to the valve drive mechanism is often insufficient, so that when the engine is running, the lubrication condition becomes very severe. In this kind of condition, when a rolling and/or sliding contact relationship is provided between the outer peripheral surface of the outer ring and the outer peripheral surface of the cam, 2 to 10 .mu.m deep peeling usually occurs on the outer peripheral surface of the outer ring, so that the life of the outer ring is shortened.
Of the technology mentioned above, in the invention described in Japanese Patent First Publication KOKAI No. H4-54312, if the lubrication condition is severe, regardless of the friction conditions, peeling cannot be sufficiently prevented.
Also, in the invention described in Japanese Patent First Publication KOKAI No. H3-199716, there is no real problem when the friction is only rolling friction, however, when sliding friction occurs, peeling cannot be sufficiently prevented.
Moreover, in the inventions described in Japanese Patent Publications KOKOKU No. H1-30008, and KOKAI No. H3-117723 to No. H3-117725, if the lubrication condition is severe, and if the operating conditions are severe, such as a large contact load, again peeling cannot be sufficiently prevented.
The bearing part of Patent Publication KOKOKU No. H2-17607 can not afford to provide enough anti-peeling property if no additional process is added.
In addition, Japanese Patent First Publications KOKAI No.52-17525 and No.56-150622, and Japanese Patent Publication KOKOKU No. 63-44505 disclose a processing method in which shots of very fine particle size are blown at high velocities on to the surface to be processed, or articles experienced such a process. However, none of these publications discloses the rolling/sliding member having enough anti-peeling property.